The present invention relates to an antenna device and a method for manufacturing an antenna device.
In the recent years, smart key systems have become quite popular in vehicles and homes. A smart key system wirelessly transmits and receives information that relates to, for example, an ID code as an electromagnetic wave. When such an ID code is collated, an owner can perform operations, for instance, to lock and unlock a door of such a vehicle or house, or to start and stop the engine without using a mechanical key. In the smart key system mentioned above, an antenna device, which has a coil antenna to transmit and receive the information, is used.
As an antenna device explained above, for instance, related technologies are disclosed in Japanese Patent Number 5050223. The antenna device that is disclosed in this Japanese Patent is configured with a first magnetic substance core, a first coil, a second magnetic substance core, and a second coil.
The first magnetic substance core is in a flat rod (bar) (rectangular bar) shape. Further, the first coil is located at an outer circumferential side of the first magnetic substance core, and one end of the first coil is connected to a first terminal. The second magnetic substance core has a toroidal closed magnetic path structure and has a configuration in which though magnetic saturation occurs at the time of the transmission of a signal radio wave, the magnetic saturation does not occur at the time of the reception of the signal radio wave. Further, the second coil is wound around the second magnetic substance core. One end of the second coil is connected to the other end of the first coil, and the other end of the second coil is connected to a second terminal.
In the configuration disclosed in Japanese Patent Number 5050223, a special magnetic substance core that is the second magnetic substance core is used, and at the same time, the second coil that is wound around the second magnetic substance core is used. As a result, a resonance frequency is adjusted. Therefore, this construction has the possibility of being complicated.
Further, in an LC (an inductor and a capacitor) resonant circuit, because a resonance frequency is defined within a narrow range, the mathematical product (L*C) of L and C (an inductor and a capacitor) can also be changed only within a narrow range. On the other hand, in regards to the first coil, a part, in which a wire is densely wound, exists. However, in that densely wound part, a change of an inductance value L per one turn can be, for instance, about 10 pH. Further, in general, a capacitor that is used in a resonant circuit also has a characteristic variation (as an example, ±5%) during manufacturing. As a result, it may become desirable to adjust an inductance value L of the first coil every 1 pH, however, this adjustment is difficult. Therefore, it becomes difficult to absorb the variation of the electrostatic capacity value C, and the variation of the product of the inductance value L and the electrostatic capacity value C, which is needed when a resonance frequency is obtained, is suppressed within a resonance range.